Snow vehicles such as snowmobiles generally ride upon one or more skis and a driven track. The track is driven by one or more drive wheels which are turned by an output shaft of an internal combustion engine.
Two-cycle internal combustion engines are the most common type of engine used to power snowmobiles. These engines have a large power output relative to their size.
As is well known in the art of two-cycle engines, an exhaust timing control valve is provided corresponding to the exhaust port of each combustion chamber or cylinder of the engine. The exhaust timing control valve is of the type which does not ever completely close the exhaust port, but moves between positions in which it partially obscures the port to varying degrees.
Generally, the timing valve is moved to a retracted position in which the valve does not obscure or obscures very little of the port when the engine speed is relatively high. In this manner, the exhaust time is lengthened. On the other hand, when the engine speed is relatively low, the valve is moved to a position in which it obscures a larger portion of the port. This delays the opening of the exhaust port and thus increases the duration of combustion. This permits higher power output with lower emissions.
A problem arises with the use of this type of exhaust timing system when the engine is mounted in a snowmobile or similar snow vehicle. Snowmobiles are often operated on very rough terrain and the intense vibrations which are transmitted through the frame may damage the exhaust timing valve system. In addition, the engine is normally mounted in an enclosed space defined by a shroud of the snowmobile in order to maintain the operating temperature of the engine. The higher temperatures within the shroud may also damage the exhaust timing valve system.
It is, therefore, a principal object of this invention to provide an improved exhaust control system for an engine powering a snowmobile or similar snow vehicle.